A soda recovery boiler is used for combusting waste liquor generated in connection with pulp manufacture, containing various sodium salts besides organic matter and water. During the operation of the boiler these salts form a smelt pool on the furnace floor, from which smelt continuously flows through smelt spouts to a dissolving tank. The smelt spouts are typically located approximately 250 mm above the level of the furnace floor. Typically, there is a smelt layer of at least approximately 300 mm on the furnace floor continuously during operation.
When the soda recovery boiler is shut down for maintenance, for example, the furnace floor remains covered with smelt. When the floor cools down the smelt solidifies and forms a hard “cake”, which must be removed by water washing or by chiseling if the aim is to clean the floor for maintenance work or inspections. The cleaning of the floor significantly extends the shutdown period, so a method and apparatus for removing smelt from the furnace by pumping have been developed in order to save time, as disclosed in the Finnish patent application no. 974206. The pumping is started in a situation where the surface of the salty smelt has reached the level of the lower edge of the smelt spout opening when the boiler is being shut down. Heating of the smelt is continued with a gas or oil flame and a spiral pump is used for the pumping.
When using a spiral pump the smelt is pumped from the furnace floor using a straight pipe having a pipe mouth at one end and a pump drive at the other end. There is a curved portion around the middle of the pipe, forming a discharge pipe through which the salty smelt is discharged from the pipe. In soda recovery boilers where the floor profile is such that the smelt pool is at its deepest close to the smelt spouts, it may not be possible to pump smelt from the deepest location of the pool with a spiral pump; rather, a considerable amount of smelt may, in many cases, remain on the furnace floor after the pumping. This extends the time needed for water washing and thus slows down the shutting down of the boiler. Another problem associated with the use of spiral pumps is that their installation requires a fairly long period of work in the immediate vicinity of the smelt spouts.